This occurs where the specimen is mounted in such a way that the optical properties (usually refractive index or coverslip thickness) of the mounted sample are not in compliance with the design requirements of the objective. This can result in severe spherical and/or chromatic aberration in the image and is one of the most common problems in confocal microscopy.

Why ‘dry’? Because if our sample is sitting in something with a higher refractive index than air, we should in principle be able to get better resolution, since the wavelength of light becomes shorter the higher the refractive index.